DN49 - No Design Switiching Regulator 5V Buck-Boost (Positive to Negative) Regulator

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No Design Switching Regulator
5V Buck-Boost (Positive-to-Negative) Regulator – Design Note 49
Ron Vinsant
Introduction
This simple, no design regulator, operates with an input
between 4.5V DC and 40V DC. It provides a –5V output
at a maximum output current of 1A to 3A depending
on input voltage.
This converter is based on the Linear Technology
LT®1074 switching regulator IC. This device needs only
a few external parts to make up a complete regulator
including thermal protection and current limit. This
design uses off-the-shelf parts for low cost and easy
availability of components. Specifications for the circuit
are in Table 1.
Circuit Description
Figure 1 shows the schematic of the circuit. For the
purpose of this explanation assume that the output
is at a constant –5V DC and that the input voltage is
greater than +4.5V DC.
At intervals of ≈10μs (100kHz) the control portion of the
LT1074 turns on the switch transistor between the VIN
and VSW pins impressing a voltage across the inductor, L1. This causes current to build up in the inductor.
The control circuit determines when to turn off the
switch during the 10μs interval to keep the output
voltage at –5V DC. When the switch transistor turns
off, the magnetic field in the inductor collapses and the
polarity of the voltage across the inductor changes to
try and maintain the current in the inductor. This current in the inductor is now directed (due to the change
in voltage polarity across the inductor) by the diode,
D1, to the load. The current will flow from the inductor
until the switch turns on again, (continuous operation)
or until the inductor runs out of energy (discontinuous
operation).
C2 is a low ESR type electrolytic capacitor that is used
in conjunction with L1 as the output filter. C5 and L2
form a post filter that reduces output ripple further.
Referring back to Figure 1, the divider circuit of R1, R2,
R3 and R4 is used to set the output voltage of the supply against an internal voltage reference of 2.21V DC.
R3, R4, C3 and C4 make up the frequency compensation
network used to stabilize the feedback loop.
Conclusion
This Design Note demonstrates a fully characterized
positive to negative converter circuit that is both simple
and low cost. This design can be taken and reliably used
in a production environment without the need for any
custom magnetics. A P.C. board layout and FAB drawing
are available from Linear Technology.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Table 1. Performance Summary (Operating Temperature Range 0°C to 50°C)
Input Voltage Range
Output
09/91/49_conv
+4.5V to +40.0V DC
Output Voltage (± 0.15V DC)
– 5.00V DC
Max Output Current At VIN = 4.5V DC
1.0A DC
Max Output Current At VIN = 40.0V DC
3.5A DC
Typical Output Ripple at IOUT = 2.5A DC
at Switching Frequency
With Optional Filter (L2 and C5)
Without Optional Filter (L2 and C5)
50mVp-p
300mVp-p
Load Regulation VIN = 4.5V DC
At IOUT = 0.1A DC to 1.0A DC
0.6%
Line Regulation ILOAD = 1A
At VIN = 4.5V DC to 40.0V DC
0.2%
A Part of EDN Magazine
–VIN
COMMON
+
$
220μF
50V
5
+VIN
-
˜)
)4
VIN
V48
R3
L
)4
VC
3
–VOUT*
R4
L
FB
C5*
39μF
35V
4
73-5$5
GND
+
3
2.74k
%
MBR745
2
C4
0.047μF
C3
˜'
R2
L
L2*
˜)
+
C2
˜'
7
–VOUT
* OPTIONAL
]
[
333
o7
OUT
R2
%$t4$)
FRONT VIEW
5
VIN
4
VSW
3
GND
2
VC
1
FB
T PACKAGE
5-LEAD PLASTIC TO-220
Figure 1. Package and Schematic Diagrams
Table 2. Parts List
REFERENCE
DESIGNATOR
QUANTITY
PCB
1
PART NUMBER
DESCRIPTION
VENDOR
003A
PCB FAB, Buck-Boost Converter
LTC
D1
1
MBR745
Diode, Schottky, 7A, 45V
Motorola
HS2
1
6038B-TT
Heat Sink
Thermalloy
VR1
1
LT1074CT
Switching Regulator, 100kHz
LTC
HS1
1
7020B-MT
Heat Sink
Thermalloy
C1
1
UPL1H221MPH
Cap, Alum Elect, Low ESR, 220μF, 50V
Nichicon
C2
1
LXF10VB272M12X30LL
Cap, Alum Elect, Low ESR, 1000μF, 10V
United Chemicon
C3
1
CKO6BX103K
Cap, Ceramic, 0.01μF, 100V
AVX
C4
1
CKO5BX473K
Cap, Ceramic, 0.047μF, 100V
AVX
C5
1
UPL1V390MAH
Cap, Alum Elect, Low ESR, 39μF, 35V
Nichicon
L1
1
CTX 25-5-52
Inductor, 25μH, 5A
Coiltronics
L2
1
CTX5-5-FR
Inductor, 5μH, 5A
Coiltronics
R1
1
MF 1/8W 2.74kΩ
Res, MF, 1/8W, 1%, 2.74kΩ
R2
1
MF 1/8W 1.82kΩ
Res, MF, 1/8W, 1%, 1.82kΩ
R3
1
CF 1/4W 5.1kΩ
Res, CF, 1/4W, 5%, 5.1kΩ
R4
1
CF 1/4W 10kΩ
Res, CF, 1/4W, 5%, 10kΩ
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn49f_conv BA/GP 0991 180K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1991
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